The present invention generally relates to wire bonding methods and semiconductor devices produced using the wire bonding methods, and more particularly to a wire bonding method which disconnects a wire which is once bonded to a pad on a substrate and newly bonds another wire on the pad and to a semiconductor device which is subjected to such a wire bonding process and has a reformed pad.
FIG. 1 shows an example of a substrate on which electronic parts such as semiconductor elements are mounted. For example, a substrate 1 is made of a ceramic material, and bonding pads 22 are provided on a surface 1A of the substrate 1. An electronic part 20 is mounted on the substrate 1 by fixing input/output pins 21 of the electronic part 20 on the bonding pads 22. On the other hand, a draw-out pad 2A is connected to the bonding pad 22, and surface pattern 23 is provided between the draw-out pad 2A and an inner layer pattern 24. Signals input/output with respect to the input/output pin 21 are transferred via the inner layer pattern 24, the surface pattern 23, the draw-out pad 2A and the bonding pads 22.
The signals which are input/output with respect to the input/output pins 21 may be changed if a modification is made in a circuit network formed in the electronic part 20. In such a case, a modification is made to change the connections so as to switch the signals which are input/output with respect to the input/output pins 21. Generally, such the modification is made by providing a wire 4-2 between a draw-out pad 2A and a relay pad 2B which is already connected to a wire 4-1, and a predetermined part of the surface pattern 23 is cut so that another signal is input/output with respect to the input/output pin 21 via the wire 4-2.
The wire 4-2 may be removed by a further modification, and still another signal may be input/output with respect to the input/output pin 21 by newly providing a wire 5-1 as indicated by a dotted line in FIG. 1. In this case, the wire 4-2 which is already bonded to the draw-out pad 2A is removed, and the wire 5-1 is newly bonded to the draw-out pad 2A.
However, the connections of the draw-out pad 2A and the relay pad 2B with the wires 4-1, 4-2 and 5-1 are normally made by a wire bonding which requires heating. As a result, a thermal stress is introduced in the substrate 1 when the connections of the plurality of wires 4-1, 4-2 and 5-1 are changed. For this reason, it is desirable that the wire bonding is made using minimum heating with respect to the substrate 1.
Conventionally, a reflow bonding process is carried out as shown in FIG. 2(a), (b) and (c).
As shown in FIG. 2(a), a pad 2 which corresponds to the draw-out pad 2A and the relay pad 2B described above is formed on the surface 1A of the substrate 1. The pad 2 is made up of a conductor material 2-1 such as copper, and a gold plating layer 2-2 which covers the conductor material 2-1. For example, the pad 2 has a thickness of approximately 20 to 30 .mu.m and a size of approximately 250 .mu.m.times.250 .mu.m. A first wire 4 which corresponds to the wires 4-1 and 4-2 described above is bonded to the pad 2.
The first wire 4 is made of a gold line having a diameter of approximately 50 to 60 .mu.m or, a gold plated copper line. The first wire 4 is bonded to the pad 2 as shown in FIG. 3.
As shown in FIG. 3, a bonding chip 32 which is provided on an arm 31 vibrated by a vibrating mechanism 30 makes contact with the first wire 4 at a bonding part 35 where the first wire 4 rests on the pad 2. The bonding part 35 is pushed by the bonding chip 32 with a predetermined pressure P, and a laser beam 33 which is converged by an optical lens 34 is irradiated on the bonding part 35. Hence, the bonding part 35 is vibrated and heated, so that the first wire 4 is bonded to the pad 2 by diffusion bonding.
When a second wire 5 which corresponds to the wire 5-1 described above needs to be newly connected to the pad 2, an external force is applied on the first wire 4 in a direction C in FIG. 2(b) so as to disconnect the first wire 4 from the pad 2. Thereafter, the second wire 5 is bonded to the pad 2 similarly as when the first wire 4 is bonded to the pad 2, as shown in FIG. 2(c).
However, when disconnecting the wire 4 from the pad 2, the surface of the pad 2 becomes damaged at a part B shown in FIG. 2(b). For this reason, when bonding the second wire 5 to the pad 2 having the damaged surface, there was a problem in that the second wire 5 may not be satisfactorily connected to the pad 2. In other words, the second wire 5 may easily disconnect from the pad 2 after the bonding, and in extreme cases, it may be impossible to satisfactorily bond the second wire 5 on the damaged surface of the pad 2. In such extreme cases, it was conventionally necessary to remove the pad 2 in its entirety and thereafter form a new pad so that the second wire 5 may be positively bonded on the new pad. But the removal of the pad 2 and the formation of the new pad in place of the pad 2 required troublesome operations which must be carried out by a skilled person, and the production yield was poor.